Dioxidomolybdenum(VI) and –tungsten(VI) complexes with tetradentate amino bisphenolates as catalysts for epoxidation

Abstract

Sixteen molybdenum and tungsten complexes with tripodal or linear tetradentate amino bisphenol ligands were studied as catalysts for the epoxidation of cis-cyclooctene, 1-octene, styrene, limonene and α-terpineol. These complexes can be divided into different categories upon key features, i.e. central metal (Mo vs. W), side-arm donor (O vs. N), hybridization of the N-donor (pyridine vs. amine), ligand geometry (tripodal vs. linear diamine) and sterical hindrance (Me vs. tert-Bu substituents in the phenol part). All complexes can catalyse selectively the epoxidation of cis-cyclooctene by tertbutylhydroperoxide whereas the activities and selectivities towards other olefins (1-octene, styrene, limonene and α-terpineol) show large differences. When H2O2 was used as an oxidant in the epoxidation of cis-cyclooctene, only two of eight Mo complexes and four of eight W complexes showed any activity. This study revealed no clear correlations between the Mo and W catalyst structures and their activities.

Simple Hybrids Based on Mo or W Oxides and Diamines: Structure Determination and Catalytic Properties

“…Comparative studies for the isostructural W and Mo analogues 1 and 4, respectively, indicated superior performances for the molybdenum catalyst using H 2 O 2 or tbhp. Several literature studies reported higher catalytic activity of molybdenum compounds than the tungsten analogues using tbhp [18,22,[28][29][30][31][32][33][34][35] or H 2 O 2 , [22,28,35,36] as verified in the present work. Mo versus W analogues may possess different M = O bond strengths (which may be weaker for molybdenum counterparts) [37] On the other hand, the influence of the type of oxidant on the types of active species formed in situ for olefin epoxidation in the presence of molybdenum catalysts was demonstrated in theoretical studies [31,38].…”

The Syntheses and Vibrational Spectra of 16O‐ and 18O‐Enriched cis‐MO2 (M=Mo, W) Complexes

“…The mass and IR spectra (see below) show that the addition of either H 2 [ 16 O] or H 2 [ 18 O] in the reaction mixture leads to the clean formation of the respective isotopologue which demonstrates that the formation of dioxidotungsten centre is caused by the hydrolysis reaction wherein the included water acts as the oxide source. The molybdenum complex Mo 16 O 2 (L) was prepared by the reaction of sodium molybdate with the ligand precursor H 2 L in an acidic methanol solution . The 18 O enriched complex Mo 18 O 2 (L) was obtained straightforwardly by heating Mo 16 O 2 (L) with an excess of H 2 [ 18 O] in methanol solution and allowing the complex to crystallize upon cooling (Scheme b).…”

“…The molybdenum complex Mo 16 O 2 (L) was prepared by the reaction of sodium molybdate with the ligand precursor H 2 L in an acidic methanol solution. [26] 13 C NMR spectroscopy, which show expected chemical shifts for the octahedral complexes with C 2 rotation axis. [13,25] The isotopic purity of the compounds was unequivocally confirmed by mass spectrometry.…”

“…[6,7,12,23,24] We have earlier prepared the structurally identical complexes of tungsten and molybdenum using N,N'-bis(2-hydroxy-3,5dimethylbenzyl)-N,N'-dimethylethane-1,2-diamine (H 2 L) ligand with a natural isotope distribution ( Figure 1). [13,25,26] Both these complexes show four strong to medium intensity absorptions in the region typical for the cis-MO 2 moiety. These are 959(m), 934(vs), 899 (vs) and 868(m) cm -1 for WO 2 (L) and, respectively, 949(m), 914(s), 903(vs) and 868(m) cm -1 for MoO 2 (L).…”

“…To date, numerous dioxidotungsten VI and ‐molybdenum VI complexes with tetradentate amine phenolate ligands have been prepared and their IR spectra have been reported ,,,,. We have earlier prepared the structurally identical complexes of tungsten and molybdenum using N,N ′‐bis(2‐hydroxy‐3,5‐dimethylbenzyl)‐ N,N ′‐dimethylethane‐1,2‐diamine (H 2 L ) ligand with a natural isotope distribution (Figure ) ,,. Both these complexes show four strong to medium intensity absorptions in the region typical for the cis ‐MO 2 moiety.…”